As you're new I suggest you put off opening a shop and stick to learning for now. Do a few tuts, learn some new techniques and find ways to implement these techniques tastefully into your work. Once you improve some more you will be able to prosper with a shop, though I'd still recommend only charging vBux instead of actual money as you've got a long way to go before people will be happy to part with their money for your services.
Take a look here
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You must login or register to view this content. for some good tuts. (If you can't see any then change the time filter from "All Time" to "1 Month") Pick some at random if you want and follow them step by step. When you've completed 5 or so tuts, take a quick look back over those 5 and then make an original piece from scratch with no help at all, using what ever techniques you've learned from those tuts. Post this new piece in the gfx section here and people will gladly help you out by giving cnc.
After that, go back and do 5 more tuts but this time look at where you could improve them by thinking back to the earlier tuts you've done and the cnc people have given you. With this you are both learning new techniques and ways to improve by using old techniques. When you've completed this sig, post it here from more cnc. Take note of the cnc, improve the piece if need be and then start again with another 5 tutorials. By now you should be getting the hang of things and no doubt you'll be able to see the improvement yourself.